Films that allow passage of gases at moderate to high transmission rates are often called breathable. The gases most commonly used to demonstrate a film's breathability are water vapor (also referred to herein as moisture vapor or moisture) and oxygen. The moisture and oxygen transmission tests measure the mass or volume of a gas transported across the cross-section of a film in a given unit of time at a defined set of environmental conditions. Breathable films can be classified as microporous films or monolithic films (which are not porous).
A breathable film can be laminated onto a nonwoven substrate to form a vapor-permeable, substantially water-impermeable multilayer article. A vapor-permeable, substantially water-impermeable multilayer article can refer to an article that allows the passage of a gas but substantially does not allow the passage of water.